1. Field of the Invention
The present invention relates to subsea pipelines and connectors and/or fittings used in the pipelines. Some embodiments relate to fittings to combine and/or redirect flow in the pipelines. More particularly, the invention relates to a pipeline using the fitting to facilitate pigging and/or cleaning of pipelines.
2. Description of Related Art
Pipeline systems (e.g., subsea pipeline systems) are a maze of connected and interconnected pipeline segments that transport hydrocarbons or related products from one point to another. The pipeline connection points initiate at the wellhead or other sources (e.g., refineries, plants, vessels, or production vessels) and connect to manifolds, offshore structures, onshore facilities, intermediate processors, and, eventually, to final processing and distribution of the products. Due to the expenses and flow issues of running dedicated pipelines from the initiating point direct to processing stations, shorter pipeline segments are joined to existing or larger pipelines to create the product pipeline from the source (e.g., the initiating wellhead) to the market (e.g., onshore stations).
Pipeline interconnects have been accommodated by placing tee-shaped fittings with a perpendicular intersect into a pipeline to create what has been known as a branch or lateral connection point. These connections provide a means of combining the product from one line with another. However, these connect ions do not allow or the passage of certain pigs used for cleaning and inspection, sometimes referred to as solid-mandrel, trained, or intelligent pigs. Combining the flow of two pipelines into one line to allow for pigging by all common forms of pigs requires the use of a fitting known as a piggable wye.
Piggable wye fittings have relatively shallow intersect angles, typically about 30° or less, which allow for fairly smooth transmission of long pigs and pig trains from either of the two inlet bores through the intersect and out through a common exhaust. However, if a pig is ever run into the piggable wye in a reverse direction, the pig will jam in the intersect of the wye, abnormally shutting down the pipeline.
Reverse flow of a pipeline is becoming increasingly more common. To this end, the ability to pig in the reverse flow direction is becoming a more common consideration for pipeline design. Thus, a piggable wye fitting that can be configured to suitably pass all forms of pigs in normal (e.g., combining) flow conditions and in reverse (e.g., directed) flow conditions is desirable.
The layout of a field development or pipeline system in which production from a subsea well flows through a subsea pipeline, or flowline, to a host platform may be referred to as a tie-back or subsea tie-back. When certain physical and chemical conditions are anticipated to exist in the production stream that will be transported in the tie-back flowline, two pipelines have been installed to join the subsea well with the host platform to form a pigging loop.
The ability to pig a subsea tie-back may be needed to effectively send or launch a pig from the host platform via the first pipeline to the subsea well or to the proximity of the well. The pig may be propelled back to the host platform via the second pipeline. Such a pigging operation allows the operator to help keep the pipeline system clear of debris and or harmful matter as well as allow the operator to perform other operational, inspection, and maintenance procedures.
Having a tie-back pipeline configured in a pigging loop has typically required the installation of two pipeline runs from the host platform to the subsea wellhead or in near proximity to the wellhead. Both of these pipelines may require risers or lengths of pipe routed from the sea floor to the host platform. On the host platform, each of the pipelines is typically configured with a pig launcher and/or a pig receiver. Such a host platform must be designed with enough physical space to allow placement of the required pipe runs and pig launchers/receivers for both pipelines.
In addition to the physical space required on the host platform needed to accommodate the pipe runs and pig launcher/receivers, the host platform must be designed with enough load carrying capacity to support the two pipeline or flowline risers. If the host platform is a floating structure, this additional load carrying capacity may be accommodated by designing and building the host platform with additional buoyancy. If the host platform is a template structure bearing its load on the seafloor, the structure must be designed with appropriately sized deck beams and supporting members as well as larger and adequately braced platform legs. Such host platforms may be costly to manufacture, install, and/or maintain. Thus, piggable pipelines that reduce the load carrying capacity and physical space required on the host platforms are desirable. In certain embodiments, a single pipeline pigging loop (e.g., a pipeline loop that allows the pig to be propelled back to the host platform in the same pipeline) may reduce the load carrying capacity and physical space required on the host platform.
In some situations, the pipeline system may have more than one pipeline branch that requires pigging. Certain pigs may not be usable in such pipeline systems. For example, the pigs may not be able to negotiate a “T” fitting that connects the pipeline branches. A separate pig launcher and receiver may be needed to pig each pipeline branch. Using multiple pig launchers and receivers results in expensive, voluminous, heavy launcher and receiver barrels, and their associated piping, to be installed for multiple pipeline branches. Thus, a pipeline system that allows the pig to be propelled through multiple branches from a single pig launcher/receiver is desirable.